On 15 Jun 2022, at 12:15, Xianting Tian wrote: > 在 2022/6/15 下午9:55, Zi Yan 写道: >> On 15 Jun 2022, at 2:47, Xianting Tian wrote: >> >>> 在 2022/6/14 上午8:14, Zi Yan 写道: >>>> On 13 Jun 2022, at 19:47, Guo Ren wrote: >>>> >>>>> On Tue, Jun 14, 2022 at 3:49 AM Zi Yan wrote: >>>>>> On 13 Jun 2022, at 12:32, Guo Ren wrote: >>>>>> >>>>>>> On Mon, Jun 13, 2022 at 11:23 PM Zi Yan wrote: >>>>>>>> Hi Xianting, >>>>>>>> >>>>>>>> Thanks for your patch. >>>>>>>> >>>>>>>> On 13 Jun 2022, at 9:10, Xianting Tian wrote: >>>>>>>> >>>>>>>>> Commit 787af64d05cd ("mm: page_alloc: validate buddy before check its migratetype.") >>>>>>>>> added buddy check code. But unfortunately, this fix isn't backported to >>>>>>>>> linux-5.17.y and the former stable branches. The reason is it added wrong >>>>>>>>> fixes message: >>>>>>>>> Fixes: 1dd214b8f21c ("mm: page_alloc: avoid merging non-fallbackable >>>>>>>>> pageblocks with others") >>>>>>>> No, the Fixes tag is right. The commit above does need to validate buddy. >>>>>>> I think Xianting is right. The “Fixes:" tag is not accurate and the >>>>>>> page_is_buddy() is necessary here. >>>>>>> >>>>>>> This patch could be applied to the early version of the stable tree >>>>>>> (eg: Linux-5.10.y, not the master tree) >>>>>> This is quite misleading. Commit 787af64d05cd applies does not mean it is >>>>>> intended to fix the preexisting bug. Also it does not apply cleanly >>>>>> to commit d9dddbf55667, there is a clear indentation mismatch. At best, >>>>>> you can say the way of 787af64d05cd fixing 1dd214b8f21c also fixes d9dddbf55667. >>>>>> There is no way you can apply 787af64d05cd to earlier trees and call it a day. >>>>>> >>>>>> You can mention 787af64d05cd that it fixes a bug in 1dd214b8f21c and there is >>>>>> a similar bug in d9dddbf55667 that can be fixed in a similar way too. Saying >>>>>> the fixes message is wrong just misleads people, making them think there is >>>>>> no bug in 1dd214b8f21c. We need to be clear about this. >>>>> First, d9dddbf55667 is earlier than 1dd214b8f21c in Linus tree. The >>>>> origin fixes could cover the Linux-5.0.y tree if they give the >>>>> accurate commit number and that is the cause we want to point out. >>>> Yes, I got that d9dddbf55667 is earlier and commit 787af64d05cd fixes >>>> the issue introduced by d9dddbf55667. But my point is that 787af64d05cd >>>> is not intended to fix d9dddbf55667 and saying it has a wrong fixes >>>> message is misleading. This is the point I want to make. >>>> >>>>> Second, if the patch is for d9dddbf55667 then it could cover any tree >>>>> in the stable repo. Actually, we only know Linux-5.10.y has the >>>>> problem. >>>> But it is not and does not apply to d9dddbf55667 cleanly. >>>> >>>>> Maybe, Gregkh could help to direct us on how to deal with the issue: >>>>> (Fixup a bug which only belongs to the former stable branch.) >>>>> >>>> I think you just need to send this patch without saying “commit >>>> 787af64d05cd fixes message is wrong” would be a good start. You also >>>> need extra fix to mm/page_isolation.c for kernels between 5.15 and 5.17 >>>> (inclusive). So there will need to be two patches: >>>> >>>> 1) your patch to stable tree prior to 5.15 and >>>> >>>> 2) your patch with an additional mm/page_isolation.c fix to stable tree >>>> between 5.15 and 5.17. >>>> >>>>>> Also, you will need to fix the mm/page_isolation.c code too to make this patch >>>>>> complete, unless you can show that PFN=0x1000 is never going to be encountered >>>>>> in the mm/page_isolation.c code I mentioned below. >>>>> No, we needn't fix mm/page_isolation.c in linux-5.10.y, because it had >>>>> pfn_valid_within(buddy_pfn) check after __find_buddy_pfn() to prevent >>>>> buddy_pfn=0. >>>>> The root cause comes from __find_buddy_pfn(): >>>>> return page_pfn ^ (1 << order); >>>> Right. But pfn_valid_within() was removed since 5.15. So your fix is >>>> required for kernels between 5.15 and 5.17 (inclusive). >>>> >>>>> When page_pfn is the same as the order size, it will return the >>>>> previous buddy not the next. That is the only exception for this >>>>> algorithm, right? >>>>> >>>>> >>>>> >>>>> >>>>> In fact, the bug is a very long time to reproduce and is not easy to >>>>> debug, so we want to contribute it to the community to prevent other >>>>> guys from wasting time. Although there is no new patch at all. >>>> Thanks for your reporting and sending out the patch. I really >>>> appreciate it. We definitely need your inputs. Throughout the email >>>> thread, I am trying to help you clarify the bug and how to fix it >>>> properly: >>>> >>>> 1. The commit 787af64d05cd does not apply cleanly to commits >>>> d9dddbf55667, meaning you cannot just cherry-pick that commit to >>>> fix the issue. That is why we need your patch to fix the issue. >>>> And saying it has a wrong fixes message in this patch’s git log is >>>> misleading. >>>> >>>> 2. For kernels between 5.15 and 5.17 (inclusive), an additional fix >>>> to mm/page_isolation.c is also needed, since pfn_valid_within() was >>>> removed since 5.15 and the issue can appear during page isolation. >>>> >>>> 3. For kernels before 5.15, this patch will apply. >>> Zi Yan, Guo Ren, >>> >>> I think we still need some imporvemnt for MASTER branch, as we discussed above, we will get an illegal buddy page if buddy_pfn is 0, >>> >>> within page_is_buddy(), it still use the illegal buddy page to do the check. I think in most of cases, page_is_buddy() can return false,  but it still may return true with very low probablity. >> Can you elaborate more on this? What kind of page can lead to page_is_buddy() >> returning true? You said it is buddy_pfn is 0, but if the page is reserved, >> if (!page_is_guard(buddy) && !PageBuddy(buddy)) should return false. >> Maybe show us the dump_page() that offending page. >> >> Thanks. > > Let‘s take the issue we met on RISC-V arch for example, > > pfn_base is 512 as we reserved 2M RAM for opensbi, mem_map's value is 0xffffffe07e205000, which is the page address of PFN 512. > > __find_buddy_pfn() returned 0 for PFN 0x2000 with order 0xd. > We know PFN 0 is not a valid pfn for buddy system, because 512 is the first PFN for buddy system. > > Then it use below code to get buddy page with buddy_pfn 0: > buddy = page + (buddy_pfn - pfn); > So buddy page address is: > 0xffffffe07e1fe000 = (struct page*)0xffffffe07e26e000 + (0 - 0x2000) > > we can know this buddy page's address is less than mem_map(0xffffffe07e1fe000 < 0xffffffe07e205000), > actually 0xffffffe07e1fe000 is not a valid page's address. If we use 0xffffffe07e1fe000 > as the page's address to extract the value of a member in 'struct page', we may get an uncertain value. > That's why I say page_is_buddy() may return true with very low probablity. > > So I think we need to add the code the verify buddy_pfn in the first place: > pfn_valid(buddy_pfn) > +DavidH on how memory section works. This 2MB RAM reservation does not sound right to me. How does it work in sparsemem? RISC-V has SECTION_SIZE_BITS=27, i.e., 128MB a section. All pages within a section should have their corresponding struct page (mem_map). So in this case, the first 2MB pages should have mem_map and can be marked as PageReserved. As a result, page_is_buddy() will return false. For flatmem, IIRC, the valid addresses should be aligned to MAX_ORDER_NR_PAGES. This means first 4MB (assuming MAX_ORDER is 11 on RISC-V) should not be on buddy allocator and hence this issue does not happen in the first place. But correct me if I am wrong. >>> I think we need to add some code to verify buddy_pfn in the first place. >>> >>> Could you give some suggestions for this idea? >>> >>>>>>>>> Actually, this issue is involved by commit: >>>>>>>>> commit d9dddbf55667 ("mm/page_alloc: prevent merging between isolated and other pageblocks") >>>>>>>>> >>>>>>>>> For RISC-V arch, the first 2M is reserved for sbi, so the start PFN is 512, >>>>>>>>> but it got buddy PFN 0 for PFN 0x2000: >>>>>>>>> 0 = 0x2000 ^ (1 << 12) >>>>>>>>> With the illegal buddy PFN 0, it got an illegal buddy page, which caused >>>>>>>>> crash in __get_pfnblock_flags_mask(). >>>>>>>> It seems that the RISC-V arch reveals a similar bug from d9dddbf55667. >>>>>>>> Basically, this bug will only happen when PFN=0x2000 is merging up and >>>>>>>> there are some isolated pageblocks. >>>>>>> Not PFN=0x2000, it's PFN=0x1000, I guess. >>>>>>> >>>>>>> RISC-V's first 2MB RAM could reserve for opensbi, so it would have >>>>>>> riscv_pfn_base=512 and mem_map began with 512th PFN when >>>>>>> CONFIG_FLATMEM=y. >>>>>>> (Also, csky has the same issue: a non-zero pfn_base in some scenarios.) >>>>>>> >>>>>>> But __find_buddy_pfn algorithm thinks the start address is 0, it could >>>>>>> get 0 pfn or less than the pfn_base value. We need another check to >>>>>>> prevent that. >>>>>>> >>>>>>>> BTW, what does first reserved 2MB imply? All 4KB pages from first 2MB are >>>>>>>> set to PageReserved? >>>>>>>> >>>>>>>>> With the patch, it can avoid the calling of get_pageblock_migratetype() if >>>>>>>>> it isn't buddy page. >>>>>>>> You might miss the __find_buddy_pfn() caller in unset_migratetype_isolate() >>>>>>>> from mm/page_isolation.c, if you are talking about linux-5.17.y and former >>>>>>>> version. There, page_is_buddy() is also not called and is_migrate_isolate_page() >>>>>>>> is called, which calls get_pageblock_migratetype() too. >>>>>>>> >>>>>>>>> Fixes: d9dddbf55667 ("mm/page_alloc: prevent merging between isolated and other pageblocks") >>>>>>>>> Cc: stable@vger.kernel.org >>>>>>>>> Reported-by: zjb194813@alibaba-inc.com >>>>>>>>> Reported-by: tianhu.hh@alibaba-inc.com >>>>>>>>> Signed-off-by: Xianting Tian >>>>>>>>> --- >>>>>>>>> mm/page_alloc.c | 3 +++ >>>>>>>>> 1 file changed, 3 insertions(+) >>>>>>>>> >>>>>>>>> diff --git a/mm/page_alloc.c b/mm/page_alloc.c >>>>>>>>> index b1caa1c6c887..5b423caa68fd 100644 >>>>>>>>> --- a/mm/page_alloc.c >>>>>>>>> +++ b/mm/page_alloc.c >>>>>>>>> @@ -1129,6 +1129,9 @@ static inline void __free_one_page(struct page *page, >>>>>>>>> >>>>>>>>> buddy_pfn = __find_buddy_pfn(pfn, order); >>>>>>>>> buddy = page + (buddy_pfn - pfn); >>>>>>>>> + >>>>>>>>> + if (!page_is_buddy(page, buddy, order)) >>>>>>>>> + goto done_merging; >>>>>>>>> buddy_mt = get_pageblock_migratetype(buddy); >>>>>>>>> >>>>>>>>> if (migratetype != buddy_mt >>>>>>>>> -- >>>>>>>>> 2.17.1 >>>>>>>> -- >>>>>>>> Best Regards, >>>>>>>> Yan, Zi >>>>>>> >>>>>>> -- >>>>>>> Best Regards >>>>>>> Guo Ren >>>>>>> >>>>>>> ML: https://lore.kernel.org/linux-csky/ >>>>>> -- >>>>>> Best Regards, >>>>>> Yan, Zi >>>>> >>>>> -- >>>>> Best Regards >>>>> Guo Ren >>>>> >>>>> ML: https://lore.kernel.org/linux-csky/ >>>> -- >>>> Best Regards, >>>> Yan, Zi >> -- >> Best Regards, >> Yan, Zi -- Best Regards, Yan, Zi